1. Field of the Invention
The present invention relates to a cage rotor of an induction motor comprising a reinforcing member for preventing an end ring from being deformed at high-speed rotation.
2. Description of the Related Art
In recent years, especially in the field of machine tools, in order to improve the machining ability of a machine, the high-speed operation and high output of a spindle motor for driving a main spindle has progressed. Some induction motors generally used as spindle motors have a cage rotor which is formed by integrally molding by a casting process a plurality of conductive rods to be arranged in a plurality of through holes formed on laminated cores near their outer peripheries and extending in its axial direction and conductive portions comprising a pair of end rings to connect the conductive rods to each other at both the axial ends of the laminated cores. The increase in the operating speed of such an induction motor with such a cage rotor can cause the end rings to be distorted or broken due to the centrifugal force during high-speed rotation.
To deal with this problem, as prior art, separately formed reinforcing members which were made from high-rigidity material such as iron or stainless steel are fixedly mounted on a cage in a manner such that it surrounds the end rings so that the deformation of the end rings may be prevented. In this case, the term "deformation" of the end ring means a state where shape or condition of an end ring is temporarily or permanently changed due to elastic deformation, plastic deformation, breakdown, rupture, or the like.
As an end ring reinforcing member for a high-speed cage rotor, known is an annular element having a virtually L-shaped section which comprises a cylindrical wall portion for covering the outer peripheral surface of the cylindrical portion of the end ring and an annular end wall portion for covering the outer surface of the end ring. According to this reinforcing member, deformation (deformation in a radial direction) of the end ring caused by centrifugal force is prevented by its cylindrical wall portion. However, since the distal end of the cylindrical wall portion is a free end although the proximal end is coupled to the outer periphery of the end wall portion, the cylindrical wall portion itself is distorted outward in the radial direction when the shaft is rotated at a high speed, with the result that the cylindrical wall portion cannot support the end ring any longer.
As means for solving the above problem, a cage rotor comprising an end ring reinforcing member constituted of an annular element having a U-shaped section with an open in one direction (its opening side faces the rotating shaft side) is disclosed, in Japanese Unexamined Patent Publication No. 6-105511. This cage rotor will be described below with reference to FIG. 6A and FIG. 6B which is a sectional view along a line 2--2 in FIG. 6A. This reinforcing member 122 integrally comprises a cylindrical portion 126 for covering the outer peripheral surface of an end ring 120, an end wall portion 130 extending inward from one edge of the cylindrical portion 126 in the radial direction to cover the outer end face of the end ring 120 in the axial direction, and a multi-holed wall portion 128 extending inward from the other edge of the cylindrical portion 126 in the radial direction and interposed between a laminated core 114 and the end ring 120.
A plurality of holes 132 are formed in the multi-holed wall portion 128 corresponding to a plurality of holes 116 of the laminated core 114 through which conductive rods 118 pass. With the reinforcing member 122 arranged at both ends of the laminated core 114 in the axial direction so that the holes 116 of the laminated core 114 and the holes 132 of the multi-holed wall portion 128 are aligned, the conductive rods 118 and the end ring 120 are integrally molded by a casting process. In this manner, each reinforcing member 122 is fixedly connected to the laminated core 114 and the end ring 120.
According to such an arrangement, since both the ends of the cylindrical portion 126 of the reinforcing member 122 are supported through the end wall portion 130 and the multi-holed wall portion 128, the reinforcing member 122 itself is not deformed even during an extremely high-speed rotation such as several tens of thousands revolutions per minute. The ring 120 can thus be prevented from being deformed by centrifugal force.
The reinforcing member 122 for a cage rotor disclosed in Japanese Unexamined Patent Publication No. 6-105511 is formed into the above shape from a round rod of iron, stainless steel, or the like by mechanical process such as cutting or grinding. In this case, an annular blank made of a rod is bored along its internal wall of the center hole so that an annular recess which opens toward the inner peripheral wall is formed. In this manner, an integrated body having a virtually U-shaped section and comprising the cylindrical portion 126, the end wall portion 130, and the multi-holed wall portion 128. In addition, a plurality of holes 132 are formed in the multi-holed wall portion 128.
In the above shaping process, especially when a high-rigidity metal material such as stainless steel is used, it is hard to maintain a machining accuracy within an allowable range in the machining stage where the annular recess which opens toward the inner peripheral wall of the reinforcing member 122 is bored. Thus, the manufacturing cost of the reinforcing member required to maintain necessary machining accuracy tends to increase. For a cage rotor required to operate at an extremely high speed such as several tens of thousands revolutions per minute (for example, 30,000 rpm or more, although the number varies depending on the outer diameter of the rotor), the reinforcing member such as the reinforcing member 122 is preferred to be used regardless of the cost. However, in a high-speed region (for example, 15,000 rpm to 30,000 rpm, although the number varies depending on the outer diameter of the rotor;) not belonging to an extremely high-speed region, the reinforcing member such as the reinforcing member 122 results in an excessive equipment investment, giving rise to the problem of the increase in cost.